Today, video games may be played by a user watching a display screen of a head-mounted display (called the HMD hereunder) worn on the head and connected to a game machine (e.g., see PTL 1). If the position and posture of the user's head are acquired so that images of a virtual world are presented to the user in such a manner that the field of view is varied in accordance with the acquired direction of the user's face for example, it is possible to produce a situation where the user feels as if he or she is actually in the virtual world. Generally, the user's position and posture are acquired through analysis of visible or infrared light images captured of the user or on the basis of measurements taken by motion sensors inside the HMD, for example.